MongoDB Performance for more data than memory

http://www.colinhowe.co.uk/2011/02/23/mongodb-performance-for-data-bigger-than-memor/

by chaostheory • 15 years ago

34 20 15 years ago

colinhowe.co.uk

antirez5• 15 years ago

I don't think it's a MongoDB fault, it is simply true with everything that is serving data bigger than available memory. Of course there is also no solution to the problem as the disk is slower... unless your working set still fits you are going to pay the added latency.

colinhowe2• 15 years ago

Perhaps I wasn't clear... it's not a fault at all, in fact MongoDB is behaving exactly how you'd expect and hope when the dataset gets too large. I just couldn't find anything out there to confirm this :)

antirez5• 15 years ago

Right, makes sense and experimenting with different systems is the only sane way to make a solid idea about the different solutions... instead of asking on twitter what NoSQL database to use ;)

Btw it's worth noting that there are on-disk databases that will behave pathologically in scenarios where the available memory starts to get seriously smaller than dataset. For instance I saw instances of MySQL that once the data set size reached a given threshold the performance started degradating exponentially. This can probably be avoided.

nkurz2• 15 years ago

Do you have a sense of how Redis fares in a similar test? I realize there are lots of differences, but I'd be interested in comparison with the same test set, both to see performance drop off as memory fills, and to compare efficiency of memory usage.

antirez5• 15 years ago

In Redis performance is the same regardless of the number of keys, there is no such an issue. But of course in Redis is easy because everything is in memory, and we use O(1) data structures for lookups.

If you use Virtual Memory things are different of course, unless you almost always hit the part of the dataset fitting in RAM. With diskstore there is also to flush things on disk from time to time, so if the site is write-heavy regardless of the amount of memory you'll see a significant slow down.

Basically we think that for write heavy apps the default (in memory) is the way to go. Actually it is a bit different than that: everybody should run Redis in memory, but some specific applications where disk-backed makes sense.

thibaut_barrere2• 15 years ago

One question I really have is: for the exact same dataset, how much RAM is used to handle it with MongoDB, Redis, MySQL, or even a column-based store.

If someone has such a benchmark already, I'd love to read it.

otterley7• 15 years ago

There's no way to know, really. When you mmap() backing stores into process memory as MongoDB and Redis do, the OS takes care of paging activity for you. And they get to compete with other processes on the system for memory pages.

InnoDB (MySQL) allows you to fix its buffer pool in RAM, so it makes decisions as to which pages to buffer instead of the OS. Sometimes it makes better decisions, sometimes it doesn't.

gvp• 15 years ago

The big problem with this question is "what's the same dataset"?

The same dataset can be structured multiple different ways. In fact, if your data structure in MongoDB is the same as it is in MySQL, then you're probably structuring it incorrectly.

By the same measure, what are you then testing for? Are you testing for lots of reads, lots of writes? What type of reads? How many servers are you testing across?

MongoDB makes it really easy to denormalize a data structure, so it's easy to "cheat" on the 99% query and perform poorly on the 1% query. This is probably what you want from a real-world performance stand-point, but you have to be really careful about factoring that into your test weightings.

Either way, I'd love to read such a benchmark as well. But to do any justice to the sheer complexity of the task, it probably needs to be a short book.

sergei• 15 years ago

The problem with Mongo's concurrency approach is that when you introduce writes to the equation (which take out an exclusive lock on the database), the system becomes unusable.

Here's a link to some of the benchmarking work I did. Ignoring the comparison to Clustrix, you can clearly see readers getting starved out when a writer does io.

http://sergeitsar.blogspot.com/2011/01/mongodb-vs-clustrix-c...

MongoDB Performance for more data than memory

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